This invention relates to the production of mineral wool insulation and more particularly, it concerns an improved furnace for melting raw materials used in the manufacture of high temperature mineral wool insulation.
In the manufacture of high temperature mineral wool insulation such as rock wool blankets for insulating boilers and other surfaces heated to temperatures of 1200.degree. F., from blast furnace slag, basalt and other low-cost, primarily siliceous raw materials, a liquid or glass melt is first formed for subsequent fiberizing to provide the insulation product. Because of the high temperatures required to melt the raw materials or rock and in part also because of the composition of the material, present melting processes are limited largely to the use of metallurgical cupolas in which the rock is placed directly onto a bed of burning coke so that on melting, the molten rock or glass drips out of the bed for collection and passage to a fiber forming station or a fiberizer.
The process presently in use is fraught with numerous problems including increasingly high costs, limited range of glass compositions capable of being used, environmental contamination and inconsistencies in the product formed. For example, the availability of coke sufficiently high in quality to withstand handling in the melting process without loss due to coke breakage and to be free of impurities which could contaminate the glass melt has steadily deteriorated with the result that increased costs of the coke itself represent substantially added costs to the known process. Irregularity in the operation of the cupola adversely affects melting and delivery rates to the fiberizer. In the known process, for example, the cupola is charged periodically at fifteen to twenty minute intervals with additional coke and rock. Such a loading cycle results in a thermal cycle in the cupola which, in turn, causes inconsistent melting and delivery rates. Because of the requirement for combustion air to pass through the coke and rock bed, all of the particulate and chemical emissions of the combustion process are passed from the cupola and require treatment for compliance with environmental standards. The requirement for passing combustion air through the bed limits the size of rock and coke particles in the sense that they must be large enough to allow passage of the combustion air. Also, there is a limit to the range of rock composition that can be effectively melted and assimilated in the stream of glass which drips from the bed.
Although many of the problems presented by the present process of melting slag and low-cost rock for the production of high temperature mineral wool insulation could be avoided by the substitution of an electric melting furnace for the cupola of the present process, a combination of the high temperatures required to melt and retain the molten rock or raw materials for delivery to the fiberizer as well as the chemically active character of the glass formed from these raw materials is severely limiting. For example, the glass formed from slag and other comparable low-cost rock materials has substantially alkaline characteristics. When heated to the temperatures required to provide a homogenous, low viscosity melt, the melt reacts with and deteriorates most types of refractory linings needed to prevent loss of heat from the melt to the exterior of the receptacle or furnace in which the melt is formed. In light of this characteristic, furnace wall cooling techniques have been attempted to maintain a temperature equilibrium by which the molten glass is contained within a lining constituted by a solidified layer of the same glass. The high heat losses experienced with this type of electric melting furnace, however, results in an economic deterrent to the use of an electric melting furnace for the cupola of the present process. While the problems associated with the cupola of the presently used process, therefore, justify serious consideration of an electric furnace melting system, particularly from the standpoint of attaining a more consistent product by virtue of the added facility for temperature controls and continuity provided by such systems, an acceptable electric melting furnace has not been available for handling low-cost materials of the type aforementioned.